Hi,
I’m planning to make some underwater blue lights using high powered LEDs.
I have underwater housings that are approximately 250mm long cylinders with a 95mm diameter.
Inside, im thinking of using the cree xpg-3 royal blue. Datasheet can be found here: https://assets.cree-led.com/a/ds/x/XLamp-XPG3.pdf
I would need to light up the field of view of video cameras underwater at night time.
Each of the LEDs mentioned can produce 777 lumens. They have a max forward voltage of 3V and driving current of 350mA.
Originally I was thinking of using 4 of these LEDs on a PCB in series, however I am concerned this may be overkill and im not sure about the battery requirements of powering a set up like this as well as the potential for overheating. Perhaps it may be better to wire them in parallel…?
I would mount the PCB on a heat sink and use an LED driver.
Any advice on whether this kind of set up might be overkill for the application, and choosing a power source would be much appreciated.
I am a complete newbie to this, so feel free to mention any other considerations that I may have missed.
Hi Sarah
Series or parallel, the number of Watts is the same.
Cheers Bob
Thanks, what about battery amperage?
Parallel: Forward voltage = 1 X individual V drop(3V). Current = 4 X individual current (1.4A)
Series: Forward voltage = 4 X individual voltage drop (12V). Current = 1 X individual current (350mA)
Assuming 4 LEDs in group.
Cheers Bob
Hi Sarah
I have not had first hand experience with these constant current devices but I think 350mA or thereabouts is a “standard” off the shelf unit. And 12V is a “standard” sort of battery.
With the need to keep to easily obtainable parts and I think you need a bit of voltage “head room” but not too much I would consider the following.
Groups of 3 LEDs in series ( 3 of 3V = 9V) driven by a 12V source via a 350mA constant current supply. The constant current supply will adjust the voltage applied to the LEDs to maintain 350mA.
Have the required number of groups connected in parallel driven by a common 12V source. The current requirement of this source would be 350mA X number of groups
You could have a 24V supply and a series/parallel combination of groups but that would be a bit messier. Best to keep in simple.
The reason for the change to 3 LEDs is that 4 in series would require about 15V supply . While this is doable it is not as common as 12V and it would be good to keep this as simple as possible.
Have a think about it
Cheers Bob
Whether or not that is overkill will depend on many factors, such as the distance of the objects to be photographed, the clarity of the water, the sensitivity of the camera (and whether it is operating in mono or colour mode) and the enclosure for the LEDs. Is there a reason for running the LEDS at 350mA - they will handle much more current than that?
When you select a LED driver it will be described as 350mA (or 700mA, if you choose that) and a wattage. The wattage determines how many LEDS you can string together in series and the driver will still be able to supply the required amperage. For instance, those LEDS are rated at 2.8V for 350mA, so they each require about 1w. So a ‘Constant current 350mA 5W driver’ would support a string of 4 LEDS at 350mA (although you might like to allow for some greater headroom than that, as the wattage rating of the LED drivers is often overstated). In other cases the LED drivers are specified as constant current and an output voltage range, eg ‘350mA DC10-18V’. This means that the driver can generate a voltage of between 10V and 18V in order to maintain the rated current of 350mA. In your case a string of 4 LEDs at 350mA would be driven at 4x2.8V so that driver would be OK. It would probably be described as a ‘3-6W’ driver. (If you choose to run them at 700mA the LED voltage goes up to 2.9V, and the wattage increases accordingly). Note that you should not select a LED driver that has a lower minimum voltage than you string requires, as it may not be able to regulate the current correctly. Similarly, do not test the driver with just one or two LEDs - always use the size of string it is designed for.
To further complicate things you need to consider the power supply available for the LED drivers, and ensure that you are reading the right figures. There is an example here of the sort of table you need in order to make the correct selection.
Hi Jeff, Sarah
Learned something to-day. I was not aware these devices actually “boosted” to the required voltage. I always thought the volts had to be supplied (plus a little bit of headroom) and the constant current device reduced or controlled this to maintain the current. I still do think some are like this and some research should be done before committing to purchase. I think some are specified a maximum voltage although as in your linked chart a voltage range is specified which should be read as min to max.
Cheers Bob
PS I try to learn something new each day so to-day is taken care of.
Hi Sarah,
It seems Bob and Jeff have already got you covered, but we have a guide to driving high power LEDs on our site which might be worth a read to fill in any queries or doubtful points you still had.
